1. Field of the Invention
This invention relates to release layers for templates used in imprint lithography, and more particularly, to release layers and the use thereof for release of templates used in step and flash imprinting processes of photocationic curable films.
2. Description of Background
Imprint lithography has emerged in various forms as a potential alternative to conventional photolithography because of its ability to print smaller features at low cost. Step and flash imprint lithography (SFIL) is a variant of imprint lithography that is amenable to the resolution and overlay requirements necessary for the fabrication of advanced semiconductor devices. In SFIL, a low-viscosity photosensitive molding material is molded between a mechanically rigid template having a relief pattern and a substrate and then is exposed to actinic radiation. The resulting hardened layer, having a three dimensional pattern, can be used as an etch mask to transfer the imprinted pattern into the substrate below.
It is generally necessary to pre-coat the template with a release agent to avoid sticking of the newly cured molding material to the template. Fluorosilanes have been widely used as release agents. The original assumption and impetus for using fluorosilanes is that by providing a low energy surface adjacent to the cured resist the adhesion energy will be minimized because the physical and chemical attraction between the two surfaces will be minimized. However, it has been found that that although the energy of the release coating is low this coating is not inert. Single and multiple cure passes are found to degrade the fluorosilane coating by lowering its fluorine concentration.
Release coating failure of fluorosilanes is believed attributable to chemical attack because of the proximity of free radicals or acid, which drive the curing reaction of the photosensitive molding material. This results not only in loss of fluorine, but can lead to higher template adhesion due to chemical bond formation of the template with the cured material. For example, for one free radical cure formulation studied, the free radical attack on the release layer lead to higher adhesion to the template than if there were no release layer present.
In another example, cured vinyl ether resists exhibited segregation of the ionic photoacid generator (PAG) to the surface of the film as determined by angle-resolved x-ray photoemission. The locally high concentration of PAG can be expected to ensure maximum consumption of monomers during polymerization as well as provide conditions for efficient attack of the release layer. As a result, the fluorosilane release coatings can fail to provide a benefit.
It is also noted that the adhesion energy of cured resists to a fluorosilane-coated template varies significantly with resist chemistry. For example, the acrylates tend to have lower adhesion (<1-2 J/m2) than the vinyl ethers (>4-5 J/m2 and often much higher).
Because it is generally desirable to reduce adhesion in order to minimize the possibility of damaging the newly cured resist layer during template removal, it would be advantageous to have a release layer that reduces the extent of polymerization and hence fracture toughness near the surface, thus facilitating crack propagation at the resist-release interface during cure. A release layer that would achieve this goal must not be itself attacked or degraded during cure. There is no mechanism for the widely used fluorosilane layer to achieve this goal.
Accordingly, there is a need in the art for improved release layers for templates used in step and flash imprint lithography. The release layers for templates must be thin, a few monolayers at most, and very smooth to avoid interfering with pattern dimension control.